Paper for use in the manufacture of electric cables and capacitors and other purposes



Nov. 6, 1962 E. KELK 3,062,912 PAPER FOR USE IN THE MANUFACTURE OFELECTRIC CABLES AND CAPACITORS AND OTHER PURPOSES Filed March 20, 1959Inventor 0004/ A Home y United States Patent PAPER FOR USE 1N THEMANUFACTURE OF ELECTRKI CABLES AND CAPACETORS AND GTHER ?URPGST.S

Eric Kellr, Harmondsworth, England, assignor to British InsulatedCailenders (fables Limited, London, England, a British company FiledMar. 20, 1950, Ser. No. 800513 Claims priority, application GreatBritain Mar. 25, 1.958 8 Claims. Cl. 174-120) This invention isconcerned with paper for use, after impregnation with an insulating oilor compound, as dielectric material. More especially it is concernedwith paper for use in the manufacture of impregnated-paperinsulatedelectric cables.

Paper is a felted fabric of vegetable fibres which for insulatingpurposes must usually have a high tensile strength to enable it to belapped or wound at high speed without breakage and, for cablemanufacture, must have good resistance to tearing and freedom frombrittleness to permit the finished cable to be bent without damaging thepaper of which the cable dielectric is built up. It should also have ahigh electrical breakdown strength, a low power factor and be of uniformthickness and density and capable of being uniformly impregnated withinsulating oil or compound. Although capacitor tissue of a thicl ness of1 mil or less is available which has been impregnated an electricalimpulse breakdown strength of about 3000 kv./cm., when tested insingle-sheet form, it has not hitherto been possible to manufacturecable insulating paper having an equivalent impulse breakdown strengthbut which has a thickness of 2 mils or more, the best 3 mil cable paperat present in use having a single sheet, impulse breakdown strength oflittle more than 2000 kv./cm.

I have now found that a paper that meets all resonable requirements ofthe cable maker as regards mechanical strength, absence of brittleness,uniformity of thickness and density and impregnability, and low powerfactor yet has an unexpectedly high electrical breakdown strength whichapproximates to that of the best capacitor tissue can be obtained froman extremely well-beaten wood pulp stock, capable of yielding a veryhigh impermeability paper, in which stock is incorporated from 2 to 20%,preferably from 5 to of very fine glass fibre.

The proportion of glass fibre to yield a paper having optimum breakdownstrength appears to be about 8%.

Between 0 and 5% the breakdown strength increases uniformly with theincrease in glass fibre content, above 5% it increases less rapidly to amaximum of about 8% and then decreases at about the same rate until thecontent reaches about 10%. Further increase up to 20% in glass fibrecontent results in a substantially uniform decrease in breakdownstrength but at a rate less steep than the rate of increase over therange 0% to 5% of glass fibre content.

The size of the glass fibre appears to be critical. High breakdownstrength is obtainable only with very fine glass fibre, that is to say,fibre having a diameter of one micron or less. It should also be short,for example, of an average length of about 0.1 mm. Fibre of thisdiameter is known in USA. as AAA grade. The incorporation in wood pulpof glass fibre obtained by treating ordinary glass wool having a fibrediameter of about 0.017 mm. in a beater fails to give a similar increasein the breakdown strength of the paper manufactured from it and resultsin a brittle sheet. To obtain the low power factor required of a cabledielectric, I prefer to use an alkali-free or low alkali content glassfibre, for example, glass fibre known in the USA. as type E.

The degree to which the wood pulp is beaten is also of great importance.I have found that this should be ice extremely well-beaten in order toobtain an increase in the breakdown strength by the incorporation ofvery fine glass fibre. By extremely well beaten I mean having aSchopper-Riegler (freeness) value closely approaching or equal to oreven higher. The addition of such fibre to pulps having a substantiallylower freeness value, eg a value of from 40 to 65 S.R. such as thosenormally used for the manufacture of cable paper, was found not to raisebut to lower the breakdown strength of the paper. I prefer to use anunbleached, water-washed sulphate pulp, preferably of first quality.

The paper made in accordance with the invention can be used in themanufacture of impregnated paper insulated electric cables byconventional methods.

Examples of the manufacture of paper in accordance with the inventionwill hereinafter be described.

In all cases the glass fibre used was of alkali free borosilicate glass(known as type E in U.S.A.) and of a diameter corresponding to thatknown as grade AAA in U.S.A. The pulp used was a first quality,unbleached, water-washed, sulphate wood pulp. The wood pulp was beatenuntil it had a Schopper-Riegler freeness value as indicated in thesecond column of the table below and the glass fibre, dispersed in lowconductivity (demineralised) water, was then mixed with the wood pulp inthe beater, 5% by weight (based on the glass fibre) of a water solublemethyl ether of cellulose, having a viscosity of 20 centipoises at 20 C.in 2% aqueous solution, being added to the suspension of glass fibres asdeflocculating agent. My preferred method of making a single and doubleply papers in accordance with the invention from the pulp containingglass fibre thus obtained is as follows. After mixing the glass fibrewith the pulp in the beater the mixture was diluted to .5% solids in thestock chest of the papermaking machine (again using low conductivitywater) and diluted to 0.3% solids in the breast box of the machine priorto sheet formation on the Fourdrinier wire.

The single ply papers were made directly on the machine by the normalprocess. Although I would normally prefer to make the two ply papers ona twin wire machine, in this case they were made on a single wiremachine by taking from the machine a wet web of paper on to a roll andlater combining it with a web of paper passing through the machine atthe first press.

Details of single ply and double ply paper made in this way are set outas Examples 1, 2 and 3 in columns ('3) 8) of the table below.

The substance (column 5) was obtained by weighing squares 5 x 5 crns. onan analytical balance. The thick ness was obtained by taking fivereadings on each square with a standard paper-makers dead-weightmicrometer and the density was calculated from substance and thickness.The mechanical properties of the papers were found to be comparable withthose of good quality cable paper.

For comparison purposes similar single and double ply papers were madein exactly the same way except that the glass fibre was omitted. Detailsof these papers are shown as Examples 4 and 5 respectively in the tablebelow.

Samples of all five papers were vacuum dried and impregnated with alight mineral oil of the kind used in oil filled cables, the oil being anaphthenic-base oil with a high proportion of aromatics derived from lowsulphur, low-wax crude oils of South American origin, subjected afterdistillation to acid and clay treatment and having a viscosity at 25 C.of 25 centistokes. The impulse strength of all of the impregnated paperswas measured on single layers of the paper from each batch using A1"diameter electrodes, the sample being immersed in the impregnant. Theimpulse strength was calculated on the thickness of the impregnatedpaper. From column (9) of the table below it will be seen that theaddition of by weight of the glass fibre to a single ply paper ofthickness about 3 mils, made from the extremely well beaten pulp, givesan increase in impulse strength of 14% and that for similar paper in twoply form the addition of 5% of the glass fibre gives an increase of 31%and of 7 /2% of the glass fibre an increase of 62%.

To show the effect of varying the beating time and varying the glassfibre content, comparative tests were carried out on papers made on alaboratory sheet making apparatus from the same glass fibre and woodpulp as used in Examples 1-5. The same impregnant was used and theimpulse strength Was measured under the same conditions. The resultsobtained are set out in the table below as Examples 6-15.

2% to not more than by weight of short, low alkali content borosilicateglass fibre of a diameter not greater than one micron, impregnated withan impregnant selected from the group consisting of mineral insulatingoils and compounds based on such insulating oils.

5. A cable in accordance with claim 4 in which the content of the glassfibre in the stock is 510%.

6. An impregnated paper insulated cable in accordance with claim 4 inwhich the wood pulp stock is beaten to a Schopper-Riegler freeness valueat least closely approaching 90.

7. An impregnated paper insulated cable comprising at least oneconductor, impregnated dielectric material surrounding said conductor,and an impervious sheath surrounding said dielectric material, in whichthe inner part of the impregnated dielectric material comprises papermade from an extremely well beaten wood pulp stock containing from 2% tonot more than 20% by Table Per No. Sub- 'llnck- Density Imperme Impulse7 Freecent of stance ness (clam ability Strcn gth Example ness Glassplies (glm (Mil) Gui-icy (kv./cm.)

Sec/mil) 90 5 1 81 3.10 1.03 45, 000 1, 830 90 5 2 69 2. 50 1. O9 240,000 2, 240 87 7% 2 90 3. 29 1. 08 9, 000 2, 770 90 O 1 70 2. 52 1. 10150, 000 1, 600 90 0 2 75 2. 96 1.00 280, 000 1, 710 93 0 1 100 3. 5 1.14 400, 000 1, S 93 5 1 84 2.9 1. 14 160, 000 3, 040 93 10 1 89 3. 0 1.19 20, 000 3. 040 93 20 1 93 3.0 1. 21 ,000 3,020 93 1 83 3. 0 1. G9 5002, 450 93 40 1 80 2. 9 1.08 80 1, 870 93 80 1 77 2. 8 1. 08 20 65 0 1 833. 1 1. 05 800 2, 100 65 5 1 84 3. 2 l. 04 400 1, 980 65 10 1 85 2. 2 1.O5 200 1, 940

It will be seen from Examples 6 to 15 that the highest impulse strengthswere obtained when the freeness value was 93 and the glass fibre contentfrom 520%.

The accompanying drawing shows in cross-section an impregnated paperinsulated power cable in which the inner part of the dielectric is ofthe paper referred to as Example 3 and the outer part is of a standardcable paper.

What I claim as my invention is:

1. An impregnated paper insulated cable comprising at least oneconductor, impregnated dielectric material surrounding said conductor,and an impervious sheath surrounding said dielectric material, in whichat least a part of the impregnated dielectric material comprises papermade from an extremely well beaten wood pulp stock containing from 2% tonot more than 20% by weight of short glass fibre of a diameter notgreater than one micron impregnated with an impregnant selected from thegroup consisting of mineral insulating oils and compounds based on suchinsulating oils.

2. A cable in accordance with claim 1 in which the content of the glassfibre in the stock is 5-10%.

3. An impregnated paper insulated cable in accordance with claim 1 inwhich the wood pulp stock is beaten to a Schopper-Riegler freeness valueat least closely approaching 90.

4. An impregnated paper insulated cable comprising at least oneconductor, impregnated dielectric material surrounding said conductor,and an impervious sheath surrounding said dielectric material, in whichat least a part of the impregnated dielectric material comprises papermade from an extremely well beaten unbleached, Water-washed, sulphatewood pulp stock containing from weight of short glass fibre of adiameter not greater than one micron impregnated with an impregnantselected from the group consisting of mineral insulating oils andcompounds based on such insulating oils and the outer part of theimpregnated dielectric material comprises paper not containing glassfibres impregnated with the same impregnant.

8. An impregnated paper insulated cable comprising at least oneconductor, impregnated dielectric material surrounding said conductor,and an impervious sheath surrounding said dielectric material, in whichthe inner part of the impregnated dielectric material comprises papermade from an extremely well beaten unbleached, water-washed, sulphatewood pulp stock containing from 2% to not more than 20% by weight ofshort, low alkali content borosilicate glass fibre of a diameter notgreater than one micron, impregnated with an impregnant selected fromthe group consisting of mineral insulating oils and compounds based onsuch insulating oils and the outer part of the impregnated dielectricmaterial comprises paper not containing glass fibres impregnated withthe same impregnant.

References Cited in the file of this patent UNITED STATES PATENTS2,093,445 Hunter Sept. 21, 1937 2,315,736 Rosch Apr. 6, 1943 2,504,744*Sproull et al Apr. 18, 1950 2,692,220 Labino Oct. 19, 1954 2,706,156Arledter Apr. 12, 1955 2,734,095 Nears et al Feb. 7, 1956 2,772,157Cilley et al Nov. 27, 1956

1. AN IMPREGNATED PAPER INSULATED CABLE COMPRISING AT LEAST ONECONDUCTOR, IMPREGNATED DIELECTRIC MATERIAL SURROUNDING SAID CONDUCTOR,AND AN IMPERVIOUS SHEATH SURROUNDING SAID DIELECTRIC MATERIAL, IN WHICHAT LEAST A PART OF THE IMPTEGNATED DIELECTRIC MATERIAL COMPRISES PAPERMADE FROM AN EXTREMELY WELL BEATEN WOOD PULP STOCK CONTAINING FROM 2% TONOT MORE THAN 20% BY WEIGHT OF SHORT GLASS FIBRE OF A DIAMETER NOTGREATER THAN ONE MICRON IMPREGNATED WITH AN IMPTEGNANT SELECTED FROM THEGROUP CONSISTING OF MINERAL INSULATING OILS AND COMPOUNDS BASED ON SUCHINSULATING OILS.